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Wave? Particles??

Wave? Particles??. Physics 100 Chapt 22. Maxwell. E. B. Light is a wave of oscillating E- and B-fields. James Clerk Maxwell. Einstein. Light is comprised of particle-like quanta called photons. h l. p = . E=hf . Who’s right??. Waves explain diffraction & interference

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Wave? Particles??

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  1. Wave? Particles?? Physics 100 Chapt 22

  2. Maxwell E B Light is a wave of oscillating E- and B-fields James Clerk Maxwell

  3. Einstein Light is comprised of particle-like quanta called photons h l p = E=hf

  4. Who’s right?? Waves explain diffraction & interference Photons explain photoelectric effect & Compton scattering

  5. Impossible to explain interference with particles With 2 slits open no light goes here Block off one slit Now light can go here

  6. Impossible to explain PE-effectand Compton scattering with waves Electron KE (electron Volts) violet blue yellow red 0.5 1.0 1.5

  7. Make an intereferencepattern with low intensity light One photon at a time goes through the two-slit apparatus The interference pattern emerges one dot at a time

  8. Wave-Particle “duality -Light behaves like a wave when it propagates through space -And as a particle when it interacts with matter

  9. Photon photography

  10. Louis de Broglie Wave-particle duality is a universal phenomenon If light behaves as particles, maybe other particles (such as electrons) behave as waves h l h p Photons: p =  l = h p particles: l =

  11. Ordinary-sized objects have tiny wavelengths 30m/s 0.2kg 6.6x10-34Js 0.2kgx30m/s h p h mv = = l = 6.6x10-34Js 6.0kgm/s = 1.1x10-34m = Incredibly small

  12. the wavelength of an electronis not so small 9x10-31 kg 6x106 m/s - h p h mv 6.6x10-34Js 9x10-31kg x 6x106 m/s = = l = 6.6x10-34Js 5.4x10-24 kg m/s = 1.2x10-10m = About the size of an atom

  13. Send low-momentum electrons thru narrow slits See a diffraction pattern characteristic of wavelength l=h/p as predicted by de Broglie

  14. Light thru a small hole “Diffraction” rings

  15. Matter waves(electrons through a crystal) “Diffraction” rings

  16. Waves thru a narrow slit y x py Dy py

  17. Waves thru a narrower slit y x py Dy wider py When the slit becomes narrower, the spread in vertical momentum increases

  18. Heisenberg Uncertainty Principle Dy Dpy > h Uncertainty in momentum in that direction Uncertainty in location If you make one of these smaller, the other has to become bigger

  19. Heisenberg tries to measure the location of an atom For better precision, use a shorter wavelength But then the momentum change is higher Dx Dpx > h

  20. Localize a baseball h Dx Dpx > Dx Dpx > h 0.2kg SupposeDx= 1x10-10m About the size of a single atom 6.6x10-34Js 1x10-10m = 6.6x10-24kgm/s Dpx > A very tiny uncertainty Dpx m 6.6x10-44Js 0.2kg Dvx > = 3.3x10-23 m/s =

  21. Localize an electron - h Dx me=9x10-31kg Dx Dpx > h Dpx > SupposeDx= 1x10-10m About the size of a single atom 6.6x10-34Js 1x10-10m = 6.6x10-24kgm/s Dpx > Huge, about 2% of c Dpx me 6.6x10-24Js 9x10-31kg Dvx > = 7x106 m/s =

  22. uncertainty is inherentin the quantum world

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